U.S. Pat. No. 4,614,244 to Wilson et al discloses a method and apparatus for registering color separation film. One algorithm disclosed in the patent is an algorithm for determining the center of a crosshair fiducial or register mark of the type often provided on the border areas of film separations. In the patented method, the computed offsets between register mark centers determines the basis for the registration of color separation films containing register marks on the image borders. This algorithm performs adequately at the accuracy standards of the method and apparatus of U.S. Pat. No. 4,614,244.
However in recent years, the accuracy requirement for register marks and sharp edged detail in general has been tightened to an error of no more than 1/2 mil in final registration accuracy. This demanding accuracy requirement of professional strippers, namely registration errors of no greater than 1/2 mil for register marks and other sharp edged detail, has required that algorithmic techniques be upgraded in accuracy.
Pending application Ser. No. 389,855, filed on Aug. 4, 1989 in the name of Wilson et al, discloses an edge based registration algorithm which is accurate enough to meet the requirements of professional strippers, and it has indeed been applied in applications involving register marks. This algorithm requires, however, a significant time to compute registration. Since the productivity of the method and apparatus increases with decreasing cycle time (algorithm time plus machine time plus operator time) it is highly desirable to have a register mark algorithm that requires minimal computation time and yet is as accurate as the edge based algorithm for use in applications where the machine operators find judicious benefit from performing some of their color film separation registration with the aid of register marks.
Although register mark applications are fewer in number than applications requiring the registration of halftone detail, there is a growing need for a specialized registrations machine which is applicable to register marks. For example, there has been a recent proliferation of films from satellite transmissions which nearly always have fiducial marks. For such specialized applications, it is beneficial to make use of a highly specialized yet very fast algorithm for the mark registration, in comparison to the use of slower, more general purpose algorithm.
The present invention is directed to an algorithmic based method and apparatus that uses a digital image gray level centroid ("center of gravity") technique to compute the center locations of symmetric crosshair register marks. This centroid technique is tailored to be fast, accurate, repeatable, and robust in the sense of being exceedingly insensitive to the presence of dust and dirt "noise" on the crosshair image.
In accordance with the invention, a gray level weighted centroid computation is performed in a manner to avoid errors caused by dust, dirt, scratches or other "noise". The x and y centroid coordinates are computed separately by using a weighting window that is perpendicular to the centroid direction. By using a unique screening process to discard contributions made by extraneous noise, highly accurate results within the requisite 1/2 mil accuracy standard are achieved, with insensitivity to noise and minimal computational time required.